The invention relates to a circuit for the control of a piezoelectric drive.
For the operation of piezoelectric drives an alternating voltage of a given frequency is required. This frequency should excite the desired resonant mode of the piezoelectric drive. The optimum operating frequency is then situated near the mechanical resonant frequency of the piezoelectric drive. Such an electrical circuit for the control of piezoelectric drives is described in the patent specification U.S. Pat. No. 5,013,982. Said specification relates to a piezoelectric drive, particularly a travelling wave motor to which two alternating voltages are applied. The electrical circuit that is used serves to control the frequency and the phase angle of the drive voltage of the piezoelectric drive. Thus, the speed of the drive is controlled.
It is an object of the invention to control a piezoelectric drive by means of an electrical circuit in such a manner that the optimum operating point and, consequently, the maximal efficiency of the piezoelectric drive are obtained independently of varying parameters such as temperature and load of the piezoelectric drive, so as to improve the efficiency of the piezoelectric drive. At the same time, a cheap and simple to realize electrical circuit for the control of the piezoelectric drive is to be provided, which circuit can readily be adapted to different piezoelectric drives.
According to the invention, the object is achieved in that an output signal of a first measurement circuit is applied to a first input of a phase detector, an output signal of a logic circuit or an output signal of a second measurement circuit is applied to a second input of the phase detector, a second output signal of the logic circuit is applied as an input signal to a final stage which serves to supply an alternating voltage to the piezoelectric drive, a loop filter processes the output signal of the phase detector and supplies a control signal to a voltage-controlled oscillator, whose output signal is supplied to the input of the logic circuit, and a delay element has been provided to adjust the frequency of the optimum operating point and efficiency of the controlled piezoelectric drive.
As a result of this, it is achieved that the piezoelectric drive always operates with maximal efficiency throughout its lifetime. For a given output power this saves electric energy and reduces the overall volume. Moreover, an improved efficiency leads to a reduced power dissipation, as a result of which the life of the piezoelectric drive is prolonged.
Furthermore, a measurement circuit is provided wherein the voltage is detected in the measurement circuit (4) by means of a sensor electrode (14) on the piezoelectric drive (1) and is the first input signal of the phase detector (5). This measurement circuit enables the phase zero crossings and thus the phase relationship of the voltage on a sensor electrode on the piezoelectric resonator to be detected.
As an alternative, a measurement circuit is provided wherein in the measurement circuit (4), the current of the piezoelectric drive (1) flows through a measurement impedance (12) and the amplified voltage across the measurement impedance is the first input signal of the phase detector (5). This measurement circuit enables the phase zero crossings and thus the phase relationship of the current through piezoelectric drive to be detected in another manner.
In another embodiment, in the measurement circuit (4), the current of the piezoelectric drive (1) flows through a zener diode (11) and the voltage across the zener diode (11) is the first input signal of the phase detector (5). This has the advantage of a cheaper yet reliable detection of the phase relationship of the current through the piezoelectric drive. This makes it possible to dispense with the operational amplifier required in the earlier described embodiment.
As an alternative, the embodiment may be one in which in the measurement circuit (4), the current of the piezoelectric drive (1) flows through two diodes (15) arranged in anti-parallel and the amplified voltage across the diodes (15) is the first input signal of the phase detector (5). This also enables the phase relationship of the current through the piezoelectric drive to be detected by means of two anti-parallel diodes.
An embodiment may also be provided in which in the form of the second measurement circuit, (13) the voltage applied to the piezoelectric drive (1) is measured. This measurement circuit enables the phase zero crossings and thus the phase relationship of the voltage applied to the piezoelectric drive to be detected.
The alternative embodiments in which the delay element (8) delays the output signal of the measurement circuit (4) or in which the delay elements (8) delays the output signal of the logic circuit (1) in the branch between the logic circuit (10) and the second input of the phase detector (5), or the output signal of the measurement circuit (13) make it possible to preset the control circuit to a given operating point as regards the resonant frequency of the piezoelectric drive (frequency offset). Both forms of a built-in delay are technically equivalent. The delay element provides the adjustment of the optimum operating point, which is situated at a frequency slightly offset with respect to the resonant frequency of the piezoelectric drive. The position of the resonant frequency of the piezoelectric drive can be found with the aid of the position of the phase zero crossing of the voltage of the first measurement circuit with respect to the position of the phase zero crossing of the output voltage of the logic circuit or the position of the phase zero crossing of the voltage applied to the piezoelectric drive. The phase difference between the two signals used for the detection of the resonant frequency of the piezoelectric drive can be shifted depending on the dead time of the delay element.
In a further embodiment, a frequency divider is arranged in the branch between the logic circuit and the phase detector. This frequency divider divides the one output signal of the logic circuit by a natural number. This has the advantage that the excitation frequency of the final stage can be several times as high as the operating frequency of the control circuit, which reduces losses as a result of distortion of harmonic waves in the final stage.
The electrical circuit in accordance with any of the embodiments just described is included in an electric shaver in order to control the piezoelectric drive of the cutters.
In a compact electrical appliance the positive effects of a high efficiency are particularly manifest because an efficient drive enables a small overall volume to be obtained. Moreover, the reduced electric power drain reduces the current consumption, which is particularly important in the case of battery-powered shavers because this permits longer mains-independent shaving or it allows the capacity of the battery and, consequently, the weight to be reduced.